Chemical vapor deposition involves directing one or more gases containing chemical species onto a surface of a substrate so that the reactive species react and form a deposit on the surface. For example, compound semiconductors can be formed by epitaxial growth of a semiconductor material on a substrate. The substrate typically is a crystalline material in the form of a disc, commonly referred to as a “wafer.” Compound semiconductors such as III-V semiconductors commonly are formed by growing layers of the compound semiconductor on a wafer using metal organic chemical vapor deposition or “MOCVD.” In this process, the chemical species are provided by a combination of gases, including one or more metal organic compounds such as alkyls of the Group III metals gallium, indium, and aluminum, and also including a source of a Group V element such as one or more of the hydrides of one or more of the Group V elements, such as NH3, AsH3, PH3 and hydrides of antimony. These gases are reacted with one another at the surface of a wafer, such as a sapphire wafer, to form a III-V compound of the general formula InXGaYAlZNAAsBPCSbD where X+Y+Z=approximately 1, and A+B+C+D. approximately 1, and each of X, Y, Z, A, B, C, and D can be between 0 and 1. In some instances, bismuth may be used in place of some or all of the other Group III metals.
In this process, the wafer is maintained at an elevated temperature within a reaction chamber. The reactive gases, typically in admixture with inert carrier gases, are directed into the reaction chamber. Typically, the gases are at a relatively low temperature, as for example, about 50° C. or below, when they are introduced into the reaction chamber. As the gases reach the hot wafer, their temperature, and hence their available energy for reaction, increases.
As used in this disclosure, the term “available energy” refers to the chemical potential of a reactant species that is used in a chemical reaction. The chemical potential is a term commonly used in thermodynamics, physics, and chemistry to describe the energy of a system (particle, molecule, vibrational or electronic states, reaction equilibrium, etc.). However, more specific substitutions for the term chemical potential may be used in various academic disciplines, including Gibbs free energy (thermodynamics) and Fermi level (solid state physics), etc. Unless otherwise specified, references to the available energy should be understood as referring to the chemical potential of the specified material.
According to U.S. Patent Publication No. 2007/0256635, CVD reactors are disclosed in which an ammonia source is activated by UV light within the reactor. In the downflow reactors shown in this application, the UV source activates the ammonia as it enters the reactor. These applicants also indicate that lower temperature reactions in their vacuum reactors can be achieved thereby.
As is shown in U.S. Patent Publication No. 2006/0156983 and other such disclosures, it is known in plasma reactors of various types that high frequency power can be applied to the electrodes therein in order to ionize at least a portion of the reactive gas to produce at least one reactive species.
It is also known that lasers can be utilized to assist in chemical vapor deposition processes. For example, in Lee et al., “Single-phase Deposition of a α-Gallium Nitride by a Laser-induced Transport Process,” J. Mater. Chem., 1993, 3(4), 347-351, laser radiation occurs parallel to the substrate surface so that the various gaseous molecules can be excited thereby. These gases can include compounds such as ammonia. In Tansley et al., “Argon Fluoride Laser Activated Deposition of Nitride Films,” Thin Solid Films, 163 (1988) 255-259, high energy photons are again used to dissociate ions from a suitable vapor source close to the substrate surface. Similarly, in Bhutyan et al., “Laser-Assisted Metalorganic Vapor-Phase Epitaxy (LMOVPE) of Indium Nitride (InN),” phys. stat.sol. (a) 194, No. 2, 501-505 (2002), ammonia decomposition is said to be enhanced at optimum growth temperatures in order to improve the electrical properties of MOVPE-grown InN films. An ArF laser is used for this purpose for photodissociation of ammonia as well as organic precursors, such as trimethylindium and the like.
The search has thus continued for improved CVD reaction processes in which reactants such as ammonia can be more effectively utilized in greater percentages and improved films can be produced at the same reactor conditions as are currently employed.